Liquid crystal displays that are truly bistable under zero voltage bias are desirable for many practical applications. There are several types of bistable displays based on liquid crystals. They all have their advantages and drawbacks. For example, there is a large body of literature on the bistable cholesteric display where the bistable states are the focal conic and the planar alignment states. Another class of bistable liquid crystal display is based on ferroelectric type liquid crystals. Here the bistable states are both homogeneous alignment states with different orientations. The difference in orientations is determined by the angle of the dipole moment and the director of the liquid crystal molecules.
Yet another class of bistable liquid crystal display is based on the twisted nematic effect in a liquid crystal display. It relies on the interplay between the elasticity of the liquid crystal and the surface anchoring conditions. These are bistable twisted nematic displays where the bistable states are both twist states. In the Berreman bistable twisted nematic liquid crystal display, the bistable twist states are zero twist and 360° twist states (See, D. W. Berreman and W. R. Heffner: J. Appl. Phys. 52 (1981) 3032; and D. W. Berreman: J. Opt. Soc. Am. 63 (1973) 1374.). Kwok et al teaches a generalization of such bistable twisted nematic displays where the bistable twist states are φ and φ+2π twist states where φ can be several fixed values, both negative and positive (See, H. S. Kwok: J. Appl. Phys. 80 (1996) 3687, T. Z. Qian, Z. L. Xie, H. S. Kwok and P. Sheng: Appl. Phys. Lett. 71 (1997) 596, Z. L. Xie and H. S. Kwok: Jpn. J. Appl. Phys. 37 (1998) 2572, and Z. L. Xie and H. S. Kwok: J. Appl. Phys. 84 (1998) 77.). These φ values have be calculated and experimentally verified. These Berreman bistable twisted nematic displays can be called 2π-BTN displays (See, D. W. Berreman and W. R. Heffner: J. Appl. Phys. 52 (1981) 3032; and D. W. Berreman: J. Opt. Soc. Am. 63 (1973) 1374.).
Durand et al teaches another variant of the bistable twisted nematic display where the bistable twist states are zero and 180° twist states (See, I. Dozov, M. Nobili and G. Durand: Appl. Phys. Lett. 70 (1997) 1179.). The switching of such display is more difficult, but not impossible, than the Berreman bistable liquid crystal displays. Again Kwok et al teaches a generalization of such π-BTN displays where the bistable twist states are φ and φ+π twist states, where φ can be one of several published values (See, H. S. Kwok: J. Appl. Phys. 80 (1996) 3687, T. Z. Qian, Z. L. Xie, H. S. Kwok and P. Sheng: Appl. Phys. Lett. 71 (1997) 596, Z. L. Xie and H. S. Kwok: Jpn. J. Appl. Phys. 37 (1998) 2572, and Z. L. Xie and H. S. Kwok: J. Appl. Phys. 84 (1998) 77.).
Yet there is another kind of bistable display, based on twisted nematic liquid crystals. It is an invention of Jones et al and is based on the bistable surface alignment conditions on an asymmetric grating surface (See, G. P. Bryan-Brown, C. V. Brown and J. C. Jones: Patent GB 9521106.6 (October 1995).). The liquid crystal molecules just outside the grating surface can be either homogeneously aligned or homeotropically aligned. This leads to a bistable alignment of the liquid crystal cell. This bistable display can be switched by the application of an electrical pulse to select either one of the surface conditions.
Further, Boyd et al (Appl. Phys. Lett. 36, 556 (1980)) presented a bistable display based on the bend-splay deformation. That display was also based on a guest-host effect with absorbing dyes and thick liquid crystal cells. The voltages needed for switching were very high and impractical.
It is therefore the object of the present invention to provide another bistable liquid crystal device.